A transconductance (Gm) amplifier generates an output current as a function of the difference between two input voltages. An input stage, such as a differential transistor pair or a class AB circuit, produces a pair of currents in response to the input voltages. A second (output) stage, such as a current mirror, generates an output current equal (or proportional) to the difference between the pair of currents.
The large signal gain of a transconductance amplifier with a differential pair input stage has a limited linear range, and flattens out substantially as the input voltage difference becomes large. The large signal gain with a class AB input stage has a limited linear range, and increases substantially as the input voltage difference becomes large. It is known that the linear range of either of the aforementioned amplifiers may be substantially increased by providing degeneration resistors in the input stage. However, the gain itself is reduced when the degeneration resistors are present. Other solutions use FETs instead of bipolar transistors in the differential pair or class AB circuit. This improves the linearity of the amplifier gain but, again, at the cost of reduced gain.
It is desirable in view of the foregoing to provide for increasing the linear range of a transconductance amplifier without reducing its gain.